Regarding his availability for Monday night's visit by the Chicago Bears, Smith noted: "It's not up to me. It's up to the doctors and there's a lot of stuff they're looking at. At this point, it's early."

Smith spoke Wednesday for the first time since a second-quarter concussion knocked him out of Sunday's 24-24 tie with the St. Louis Rams.

Smith confirmed an earlier statement from coach Jim Harbaugh that a fourth-down run is what immediately resulted in Smith's blurred vision.

"For me, the vision was the biggest thing," Smith said. "I've taken a lot of hits over the years, and for me it didn't get better. I sat down on the sideline and felt it get worse. It was not good for me and not good for the team to get back out there."

Smith noted, however, that neurologists believe a helmet-to-helmet hit from the Rams' Jo-Lonn Dunbar may have contributed to the ensuing concussion. That play occured at the start of his final series, and near the end of the first quarter. Smith confirmed he was examined between quarters by 49ers team medical director Dan Garza.

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"I felt fine after that hit, though," Smith said. "When I came up off the sneak, my eyesight was off. ... It's tough to play quarterback without your eyes and when you can't see. There was a bunch of stuff out there I couldn't make out."

Smith received medical clearance to participate in non-contact practice drills after visiting a Stanford University neurologist who works independent from the 49ers.